A present day program-controlled PBX often has the capability of transmitting a call incoming thereto over one of a plurality of routes to the remote station being called. For example, if a first such PBX is part of a private system of interconnected PBX's of which, say, a second is in the country of the remote station, and a third PBX is in another foreign country but is connected both to the first PBX and the second PBX, then the first PBX is often capable of transmitting the call to the remote station over (a) a direct tie line between the first and second PBX's or (b) tandem lines connecting the first PBX to the third and the third to the second. Further the first PBX also usually has the additional alternative of transmitting the call via direct-digit-dialing lines over a domestic common carrier telephone network and one or more foreign common carrier telephone networks.
Evidently, it is desirable to minimize the cost of making the call and, to this end, models of the Dimension.RTM. PBX available from AT&T Technologies, Inc. (formerly Western Electric Company, Incorporated), 222 Broadway, New York, N.Y. 10038, may be equipped with the feature known as Automatic Route Selection ("ARS") also available from such company. A description of ARS is given in the article "Automatic Route Selection Made Easy" appearing on pages 283-287 of the November 1979 issue of the Bell Laboratories Record. The manner in which such feature operates is that, when a PBX receives an incoming call intended to reach a remote station and the PBX can be connected to such station over each of a plurality of transmission routes, those routes are scanned to determine which of them are idle so as to be available to transmit the call, the group of idle routes is then further examined to determine which one or ones of them permit making the call at least cost, and the call is then directed by the PBX for transmission over a one such route providing the least cost.
When a subscriber at a station served by a local PBX desires to place, say, an international call through it to a remote station in another country, the subscriber usually dials all the digits necessary to complete the call through common carrier telephone networks such as the AT&T network and the network of the common carrier in the country in which the remote station is located. That is, such subscriber would usually dial "011" to indicate that the call is of international character, then two digits identifying the country in which the remote station is located, then possibly another digit identifying the city or the area in which such station is located, and then the local exchange number and individual number of that remote station. It turns out, however, that if the PBX locally serving that subscriber is provided with ARS which will select for the call the least costly available route at the time out of a plurality of potentially available routes so that, depending on circumstances, the call may be directed over any of several of such routes, then, for one or ones of such routes, not all of the digits dialed by the subscriber are necessary for completion of the call to the remote station. On the other hand, to transmit excess dialed digits is evidently inefficient and, therefore disadvantageous.